In general, there are many known ways of estimating the presence of a signal at a receiver. For decision-making in connection with messages which are modulated to discrete values, such as “−1”, “0”, or “+1”, the signal power is usually known (e.g. relative to a pilot signal), so that signal values, e.g., “−1” and “+1”, can be detected with the same reliability, i.e., zero level can be used as a dividing or threshold line. Thus, if the received statistic is below zero, decide signal value “−1”, and if it is above zero, decide signal value “+1”.
However, in some transmission systems or standards, signal power may be totally unknown at the receiver side. As an example, in HSUPA (High Speed Uplink Packet Access) which is a new feature in 3GPP (3rd Generation Partnership Project) Release 6 specification, two HSUPA downlink physical channels, Enhanced Dedicated Channel (E-DCH) Hybrid ARQ (Automatic Repeat Request) Indicator Channel (E-HICH) and E-DCH Relative Grant Channel (E-RGCH) are provided, in which message are modulated to signal values “−1”, “0”, or “+1”. The value “−1” means “negative amplitude”, the value “0” means “no power at all”, and the value “1” means “positive amplitude”. If the value “−1” or “+1” is transmitted, the receiver at a terminal device (e.g. a user equipment (UE) in 3G terminology) must be capable of separating these values without knowing what power the transmitting base station has used or if it has used any power at all.